In recent years, automobiles on which an auxiliary power supply is mounted in addition to a main battery have been developed in order to improve fuel consumption and due to concern about the environment. For example, it is possible to reduce the power generation of an alternator and improve fuel consumption by storing regenerative energy at the time of braking in the auxiliary power supply and using the regenerative energy in power supply to electrical components during driving. Furthermore, an instantaneous power interruption of electrical components and degradation of the main battery due to a momentary drop of the main battery voltage can be prevented by supplying power to a starter from the auxiliary power supply when restarting an engine after turning off the engine when stopping.
There are cases where a switching board having a function of switching the power supply mode in association with operation status of a vehicle, such as driving or turning off the engine when stopping, is installed between the main battery and the auxiliary power supply.
Conventionally, a board having a mechanical relay capable of dealing with a large current has been used as the switching board, whereas replacement of the mechanical relay with a semiconductor switching element has been proposed in order to reduce the size, prolong the lifetime, and reduce the noise. It is expected that a power semiconductor device such as a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is used as the semiconductor switching element (see Patent Document 1).
In general, a parasitic diode with a pn junction is generated between the source and the drain in switches using a MOSFET. Accordingly, a current flows through this parasitic diode from the source to the drain even if the gate is turned off, and accordingly, the current flow from the source to the drain cannot be completely shut down. In order to make it possible to shut down currents in both directions, two MOSFETs are connected in series such that the orientations of the aforementioned parasitic diodes are opposite.
However, if such a complicated circuit is designed, the structure of the switching board tends to be complicated.